Apologies for the long response time on this. I’ve known about this issue, and I recently looked into it.
OverrideMaxPerf was an unofficial kernel parameter that was not officially supported and was removed at the time you saw the regression. However, RMForcePstate is another unsupported kernel parameter that does still exist that can accomplish the same effect, assuming you know the pstate you wish to force. I emphasize that this is unsupported and may cause unintended interactions with other power features such as dynamic boost or otherwise general instability. I myself am able to use it with a multi-monitor setup to force P8 with the desktop still being responsive (but games will not have good performance as expected).
In summary, you can specify
nvidia.NVreg_RegistryDwords="RMForcePstate=8"
to force P8, but again this is unsupported.
The related ramp-down discussions have separate threads that I will not comment on here.